There, I found it, in usr/local. And it has GD installed, so that much seems to work now. Thank you!

It seems a little silly to have two different versions, but maybe that's okay. I'd still like to see if I can get the other one to work as well, but at least this is a big improvement.

Now I'm trying to install SDL, but neither version is very keen on that. The CPAN program spends a good while installing dependencies, and then tells me that "install seems impossible":

Undefined subroutine &My::Builder::Darwin::rel2abs called at inc/My/Bu+ilder/Darwin.pm line 115.
FROGGS/SDL-2.536.tar.gz
./Build -- NOT OK
Running Build test
Can't test without successful make
Running Build install
Make had returned bad status, install seems impossible
Failed during this command:
FROGGS/SDL-2.536.tar.gz : make NO

And the PPM program has never heard of SDL, although it provides something called "Alien-SDL" which I've installed but don't know what to do with.

EDIT: Well, I tried downloading the package and installing it manually, and that gave the same error about "rel2abs". Somewhat desperate, I edited the Darwin.pm file myself, which felt very very wrong. A quick googling suggested that "File::Spec" was needed, so I added that to the file. Now the testing went fine, and the install didn't seem to have any problems either. Unfortunately neither of the Perl versions acknowledges that I have installed SDL. So maybe it ended up in a third place.

I have a policy about the Perl installed with the system / OS - do not touch it. Unless my application is being bundled with the OS or needs to run on the bare OS, I install my own version of Perl. This allows my application to dictate the version of Perl, compile options, library / module versions, and so on without needing to be concerned with a patch or upgrade of the OS hosing my application, or conversely, my application hosing the OS.